The present invention is directed toward a delivery trough for use with a melting furnace and more particularly toward an ultrahigh velocity water-cooled copper trough.
Troughs for delivery of molten material have been one of the banes of designers for years. Refractory construction is expensive and short-lived in air, builds up unpredictable accretions as the "system" attempts to reach thermal steady state and therefore corrupts an otherwise steady delivery into an unsteady, wavering flow. Essentially the same criticism applies to water-cooled metal troughs, particularly since these have invariably been made of carbon or stainless steel, cooled in a conventional manner. To date, no one has been able to produce a trough capable of providing a constant, unwavering flow of material from the discharge end thereof.
As explained more fully in Applicant's U.S. Pat. No. 4,032,705 (the entire subject matter thereof being included herein by reference), Applicant has discovered that the rapid, consistent removal of large quantities of energy (in the range of 1 BTU per square inch per second) through a water-cooled metal barrier, without damage to the barrier, requires that the metal have excellent thermal conductivity and a reasonably high melting point, and be force-cooled at a constant temperature by the creation and efficient removal of steam at its back face.
Converting 1 pound of water into steam requires 967 BTU's of heat at 212.degree. F. (or 536 calories per gram at 100.degree. C.). If water can be made to present itself consistently to the area to be cooled and there to turn into steam, and then to leave the area immediately to make room for more water to arrive, a highly efficient and predictable cooling system results. The area to be cooled must, of course, be kept free of accretion to obviate the film effects which are adverse to efficient thermal transfer.
Experimentation has shown that the best way to remove the steam film as rapidly as it forms is by applying ultrahigh velocity cooling water to the back surface of the metal barrier. A cooling water velocity of at least 10 feet per second has proved to be required, and this velocity must be at the suface of the metal, not merely at the center of a substantial cooling passage of which the metal barrier is one of the walls. The preferred water cooling velocity is at least 20 feet per second. It should be readily apparent that such velocities require high flow rates through small passages, thereby generating pressure drops of the order of 20 to 60 psi, depending on the surfaces, shapes and length of the area to be cooled.
To enhance the effectiveness of this cooling, a readily workable metal of reasonable cost and melting point and high thermal conductivity is required. From a table of the physical properties of the elements, a selection of an easily workable, relatively inexpensive material with a melting point above 1,000.degree. C. and good thermal transfer capability results in the following list:
______________________________________ MELTING POINT CONDUCTIVITY ELEMENT (.degree.C.) (calgmcm/sqcm/sec/.degree.C.) ______________________________________ Chromium (Cr) 1875 0.16 Copper (Cu) 1083 0.943 Iron (Fe) 1537 0.18 Molybdenum (Mo) 2610 0.34 Nickel (Ni) 1453 0.22 Silver (Ag) 960 1.00 (for comparison) ______________________________________
Chromium, molybdenum and nickel are not really easily workable and they are relatively expensive. Furthermore, these materials have thermal conductivities which are from 3 to 5 times poorer than that of copper.
Because of the relatively low melting point of copper and the corresponding higher melting point of iron, the automatic and quite incorrect choice in the past for a water-cooled trough has been steel. This has been true even though it has a thermal transfer ability less than 1/5 that of copper. Furthermore, for a number of reasons, the water-cooled steel has a tendency to form films thereon of a highly insulating nature.
Compounding this technical felony is the fact that, to Applicant's knowledge, no attempt has been made to ensure the efficient removal of heat energy from the back face of a trough by the encouragement of steam formation, against a clean surface, made effective by the immediate removal of that steam by new cooling water moving at "ultrahigh velocity." It should be pointed out that the use of stainless steel only makes matters worse since stainless steel grades have thermal transfer abilities 16 to 24 times poorer than copper.